1. Field of the Invention
The present invention relates to carbazole compounds that can be used as a light-emitting element material. The present invention further relates to light-emitting element materials, organic semiconductor materials, light-emitting elements, light-emitting devices, lighting devices, and electronic devices each using the carbazole compound.
2. Description of the Related Art
A display device using a light-emitting element (organic EL element) in which an organic compound is used as a light-emitting substance has been developed rapidly as a next generation lighting device or display device because it has advantages that such a light-emitting element can be manufactured to be thin and lightweight, has very high response speed with respect to an input signal, and has low power consumption.
In an organic EL element, when a voltage is applied between a pair of electrodes between which a light-emitting layer is interposed, electrons and holes injected from the electrodes are recombined to form an excited state, and when the excited state returns to a ground state, light is emitted. A wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance; thus, by using different types of organic compounds as light-emitting substances, light-emitting elements which exhibit various wavelengths, i.e., various colors can be obtained.
In the case of a display device which is expected to display images, such as a display, in order to reproduce full-color images, at least light having wavelength components of three colors, i.e., red, green, and blue, is required to be obtained. In the case of a lighting device, in order to obtain high color rendering property, light having wavelength components thoroughly in the visible light region is ideally obtained. Actually, two or more kinds of light having different wavelengths are mixed to be used for lighting application in many cases. Note that it is known that by mixing light of three colors, red, green, and blue, white light emission having high color rendering property can be obtained.
Light emitted from a light-emitting substance is peculiar to the substance as described above. However, important performances as a light-emitting element, such as lifetime, power consumption, and emission efficiency, are not only dependent on a light-emitting substance but also greatly dependent on layers other than a light-emitting layer, an element structure, properties of the emission center substance and a host, compatibility between them, or the like. Therefore, it is true that many kinds of light-emitting element materials are necessary in order to show the growth of this field. For the above-described reasons, light-emitting element materials which have a variety of molecular structures have been proposed (for example, see Patent Document 1).
In particular, high-energy emission of blue light needs an emission center substance having a wide band gap and a host material having a wider band gap; therefore, it is difficult to say the variation of materials that can be used in fabricating a light-emitting element having favorable characteristics is sufficiently wide.